Findings from survey studies indicate that the average American adult obtains an inadequate amount of sleep. Chronic sleep loss contributes to accidents and related deaths, poor job/school performance and poor health. In regards to health, short sleep duration has been hypothesized to contribute to the development of obesity however, the effects of sleep loss on mechanisms contributing to obesity are poorly understood. We propose to study 24 hour energy expenditure and mediators of energy balance during sleep loss to better understand the increased risk of obesity associated with sleep loss. The specific aims of this proposal are to determine the influence of chronic sleep loss on cognitive function, metabolism, and energy balance in humans tested in an 11 day inpatient CTRC study. The proposed research addresses several themes outlined in the NIH PA NUMBER: PA-07-140 by testing the following specific hypotheses: i) chronic sleep loss will impair mood. alertness and performance levels;ii) chronic sleep loss will impair glucose tolerance, an effect related to decreases in insulin action and increases in sympathetic nervous system (SNS) activity;iii) chronic sleep loss will lead to positive energy balance promoting weight gain. We will also examine biological mediators associated with expected alterations in 24 h energy balance during sleep loss. The proposed project addresses several of the highest recommendations for research in the 2003 NIH National Center on Sleep Disorders Research Plan including i) to enhance our understanding of the impact of reduced or restricted sleep on behavior, and neurobiologic and physiologic functions;ii) to delineate processes involved in and the mechanisms underlying the influence of chronic sleep deprivation on non-neural systems;iii) to delineate the effects of sleep loss on behaviors that diminish safety.